1. Field of the Invention:
The present invention relates to an improvement in the method for making photovoltaic devices.
More particularly the present invention concerns an improvement in the method for making photovoltaic devices by sequentially accumulating plural amorphous silicon layers of different conductivities on a substrate by means of plasma reactions of material gases containing impurity gas.
2. Description of the Prior Art:
A typical example of the conventional method and its problem in the method for making photovoltaic devices utilizing sequential accumulation of plural amorphous silicon layers of different conductivities on a substrate by plasma reaction of material gases with impurity gas is elucidated with reference to FIG. 1 showing a sectional elevation view of the general photovoltaic device, and FIG. 2 showing sectional elevation view of general apparatus for making the photovoltaic device.
As shown in FIG. 1, the photovoltaic device comprises a transparent substrate 162, such as glass, a transparent conductive film 2 such as indium oxide or tin oxide, plural amorphous silicon layers 3 and a back face electrode 4 such as aluminum film. The plural amorphous silicon layers 3 comprises a p-type layer 5 formed on the transparent electrode 2, an i-type (non-doped) layer 6 formed on the p-type layer 5 and an n-type layer 7 formed on the i-type layer 6. The amorphous silicon layers 3 are accumulatedly formed by sequential plasma reactions in silane gases containing an appropriate impurity gas. The pure amorphous silicon layer is a weak n-conductivity type, and accordingly a very small amount of p-type impurity may be added to make the layer 6 a pure i-type layer. Furthermore, an impurity of IV-th group element such as Ge or Sn may be added in order to decrease optical inhibition band width of the i-type layer, so as to adjust the spectral characteristic to that of a light source to be in conjunction therewith used.
FIG. 2 shows the configuration of a conventional plasma reaction apparatus for making plural amorphous layers 3 on the substrate 162. The apparatus has a row of chambers 10a, 10b and 10c each having shutters 19a, 19b, 19c or 19d on partitions 18, 18 . . . . The chambers 10a, 10b and 10c are reaction chambers. The chambers 10a, 10b and 10c has gas inlet valves 11a, 11b and 11c gas outlet valves 12a, 12b and 12c, respectively. The valve 11a controls feeding of silane (SiH.sub.4) gas and diborane (B.sub.2 H.sub.6) gas as impurity, the valve 11b controls silane gas and the valve 11c controls silian gas and phosphine (PH.sub.3) gas as impurity, respectively, and the valves 12x, 12a, 12b and 12c are connected to vacuuming apparatus and is for controlling evacuation of the chambers 10a, 10b and 10c, respectively. The chambers 10a, 10b and 10c have electrodes 13a, 13b and 13c and electrodes 14a, 14b and 14c, respectively, which are disposed opposing each other in the chambers, and are supplied with AC voltage from an AC power source thereacross for plasma reaction.
A roller conveyer 161 is disposed throughout the row of the chambers for conveying substrates 162, 162 . . . through the row of the chambers. The shutters 19a, 19b, 19c and 19d are open only when moving the substrate step by step from the chambers 10a to the chamber 10c.
A conventional method of making the photovoltaic device by utilizing the apparatus of FIG. 2 is as follows.
Firstly, a substrate 162 having a transparent conductive film 2 is brought into the chamber 10a through the opening 17a by means of the roller conveyer 161. At this time shutters other than 19a are closed, and after putting the substrate into the chamber the roller conveyer 161 is stopped and the first shutter 19a is closed. A at this stage, all the valves 11a to 11c and 12a to 12c are closed, and no AC voltage is fed across the first and the second electrodes 13a to 13c and 14a to 14c. Then the chambers are evacuated by opening the evacuation valves 12a to 12c, and by opening the first feeding valve 11a the first reaction chamber 10a is filled with silane gas containing a small amount of diborane gas as impurity, and at this stage the AC voltage from the AC power source 15 is supplied across the electrodes 13a and 14a, thereby producing the plasma reaction to form a first silicon film 5 of p-conductivity type on the transparent conductive film 2.
After the forming of the p-type silicon layer 5, the AC voltage across the electrodes 13a and 14a is shut off, and the first reaction chamber 10a is evacuated. Thereafter, the second shutter 19b is opened and the substrate 162 is moved to the second reaction chamber 10b, and thereafter the shutter 19b is closed. Then the second reaction chamber 10b is filled with silane gas and the AC voltage is supplied across the electrodes 13b and 14b, thereby causing the plasma reaction in the second chamber, to form the second silicon layer 6 of i-conductivity type. Thereafter, the supply of AC voltage is stopped and the chamber 10b is evacuated by handling the valve 12b, and by opening the shutter 19c the substrate is moved into the third reaction chamber 10c, and the shutter 9c is closed. Thereafter, the third reaction chamber 10c is filled with silane gas containing a small amount of phosphine gas and the AC voltage is impressed across the electrodes 13c and 14c to form the plasma reaction, and the third silicon layer 7 of n-conductivity type is formed.
In the above-mentioned conventional method to form plural amorphous silicon layers of different conductivity types on a substrate, there has been the following shortcoming: The substrate moving is made after evacuation of a chamber where a silicon layer is formed, thereby the evacuation of the chamber was made each time for movement of the substrate to next chamber, and the supply of AC voltage is to be stopped also, and only one reaction chamber of the three reaction chambers is utilized at one time and other two reaction chambers are vacant. Therefore, a considerable time is wasted for the evacuations of the used gas and removing of a substrate to the next reaction chamber, and also the material gas was wasted.